1. Field of the Invention
This invention relates generally to the purification of water and, in particular, to the removal of nitrate from water using a biological treatment method and an apparatus for performing the method.
2. Description of Related Art
Pollution is a serious environmental problem for both industry and the public. Of particular concern are streams and lakes used as water supplies, since government regulations limit the amount of various substances which may be present in water. In general, there are a number of different methods for the purification of water which include mechanical treatment by sedimentation, filtration or membranes; chemical purification through use of chemicals, gases or resins; and biological treatment by mixing the water with bacteria to convert pollutants to innocuous byproducts. The following description will be directed to biological purification of water using bacteria and, in particular, to the removal of nitrate from water, and it will be appreciated by those skilled in the art that the method and apparatus of the invention may be used for other purification processes.
Nitrate is normally present in waters associated with mining (as a result of blasting activities using ammonium nitrate or dynamite) and may be mobilized in water through other industrial activities. It may also be present in ground water and surface water in agricultural areas from fertilizer use. Nitrate is also normally present in municipal and some industrial waste streams after the aerobic degradation of ammonia.
The concentration of nitrate in water is of primary concern due to potential human health impacts from water consumption. The toxicity of nitrate to humans is due to the body's conversion of nitrate to nitrite, particularly in infants. Elevated nitrite levels can cause reduced oxygen concentration in the bloodstream, a condition known as methemoglobinemia. Accordingly, the maximum allowable limit for nitrate in federal drinking water regulations has been set at 10 mg/L as nitrogen. Discharge of nitrate-containing waters to surface water is also a regulatory concern because, as a limiting nutrient, nitrate potentially can cause undesirable aquatic plant growth.
Numerous technologies have been developed and used as a means of removing nitrates from water. Ion exchange and reverse osmosis have been successfully used worldwide for over 20 years to remove or reduce nitrate and other ions to low concentrations, but their primary disadvantages are high capital and operating costs, and generation of a concentrated waste stream. Conventional biotreatment systems have also been used for many years, primarily in municipal waste water treatment plants. These technologies include sequencing batch reactors, rotating biological contactors, and packed-bed or fluidized-bed systems. While costs for conventional biotreatment systems are generally lower than those for ion exchange and reverse osmosis, costs are still substantial and a biomass waste stream must be handled.
The term "biofilm" as used herein may be defined as a layer of a biomass on a substrate. The biomass is composed of bacteria and bacterial products in which the bacteria may be of aerobic, anoxic or anaerobic type depending on the kind of purification process employed. In aerobic processes, microorganisms need oxygen to grow while with an anaerobic process, microorganisms must have an oxygen-free environment. An anoxic environment generally contains low concentrations of oxygen and does not have to be completely without oxygen as in an anaerobic environment.
In a biofilm system, microorganisms grow on fixed surfaces in the bioreactor or biotreatment cell. The biofilm grows in thickness as the microorganisms propagate, and part of the biofilm will eventually detach and new biofilm will be formed. Biofilm reactors in use today are based on different systems such as biorotors (rotating biological contactors), trickling filters, fluidized bed reactors and a stationary bioreactor wherein the substrate on which the microorganisms grow is immersed in the reactor and is stationary while the water contacts the substrate while traveling through the bioreactor to the outlet. U.S. Pat. Nos. 5,458,779 and 5,543,039 to Odegaard show a method and a reactor suitable for the purification of water by biological methods and the patents are hereby incorporated by reference. The patents discuss the many types of bioreactors and disclose a method wherein the biofilm grows on specially configured plastic carriers which are kept suspended (fluidized) in the water in the reactor.
Nitrogen present in the form of dissolved nitrate may be removed from water through the action of denitrifying bacteria (DNB), which convert the nitrate to nitrogen gas (N.sub.2). DNB are facultative anaerobes, meaning that they reduce oxygen preferentially over nitrate and will only reduce nitrate when oxygen is not readily available. Therefore, microbial nitrate reduction can only occur in anoxic or anaerobic environments. Nitrate removal from water is often preceded by nitrification in which ammonia nitrogen is aerobically oxidized to nitrate. Nitrification is not generally required if most of the nitrogen in the water is already in the nitrate form.
In the nitrate removal process, an external carbon source such as methanol is often added to enhance denitrification. The carbon source serves as an electron donor (is oxidized to CO.sub.2) while nitrate acts as the electron accepter (is reduced to N.sub.2). This reaction in which the methanol supplies electrons to produce energy is called dissimilatory nitrate reduction. Methanol is also required to supply the carbon for creating new cell mass in the reaction, called assimilatory nitrate reduction. When converted to a mass basis, about 2.5 g of methanol are required to reduce 1 g of nitrate (as nitrogen) to nitrogen gas. The nitrogen and carbon dioxide gases produced in the reactions are typically in excess of the amounts which are soluble in water, which requires that the bioreactor have a venting system. Other carbon sources such as acetic acid or sucrose may be used. Nutrients such as phosphates are also required for bacterial growth.
Any suitable DNB may be used; they are generally mesophilic organisms which prefer temperatures of 20.degree.-30.degree. C., but may also be used at lower temperatures. Denitrification processes have been designed in industry as either suspended-growth systems, in which microorganisms are suspended in a stirred liquid, or as attached-growth systems, in which DNB are attached to a porous media with the water flowing past the media in the bioreactor. Attached-growth systems (specifically packed-bed reactors) are the area of particular concern in the present application.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a biological method for removing contaminants from water and, in particular, for removing nitrates from water using denitrifying bacteria and an anoxic bioreactor.
It is a further object of the present invention to provide an anoxic bioreactor for use in removing contaminants from water and, in particular, removing nitrates from water.
Other objects and advantages of the present invention will be readily apparent from the following description.